The present invention relates to the commissioning of new and refurbished steam generation plant equipment and piping. More particularly, the present invention relates to the cleaning and conditioning of metal surfaces in the steam and water circuits associated with steam turbine generators and similar equipment at the same time that other essential commissioning activities are being performed.
Steam turbines convert thermal energy from pressurized steam into mechanical energy. This mechanical energy is commonly used to drive electric generators or gas compressors. In combined cycle power plants, the exhaust heat of a gas turbine is used to generate steam that is then used to power a steam turbine. In some types of gas turbine designs, some of the steam generated is also injected into the combustion path of the gas turbine to enhance the power output of the gas turbine.
Particulate debris may plug start-up screens installed at the inlet control valves of the turbines or clog narrow steam passages. Particulate debris may cause erosion or impact damage to both stationary and rotating components of the steam turbines. Particulate debris may also plug or damage the internal surfaces of steam valves used to control the flow of steam.
Silica oxides dissolved in steam are another critical contaminant that may be transported from the steam generation equipment and piping to steam turbines. The solubility of silica oxides in the steam is significantly increased with increasing temperatures. During the passage of steam through a steam turbine, the temperature of the steam is reduced. This will lead to a reduction of silica solubility and the deposition of silica on the internal surfaces of the steam turbine.
Another critical type of contamination that may be transported from the steam generation equipment and piping to the steam turbine are various salts. These salts include those that lead to high levels of cation conductivity in the steam condensate. Steam cation conductivity is the conductivity measured in steam condensate that has been passed through a cation exchange resin. High cation conductivity results from anions. The presence of anions in the steam will lead to the potential of stress cracking of steam turbine components. Chlorides, organic anions and sulfates in the steam pose a particular danger for the onset of stress cracking.
Other presence of other salts, such as sodium, is also monitored in the steam. The presence of sodium in the steam risks the deposition of alkalis and/or other salts. Sodium chloride and sodium hydrogen sulfate also constitute a risk of stress corrosion cracking of turbine components.
In typical prior art methods for the commissioning of new and refurbished steam turbine generator facilities, numerous methods have been employed to remove particulate contamination from the interior surfaces of the equipment and piping used to generate and transport steam to steam and gas turbines.
One such method has been practiced for many decades and involves the pressurization of the steam generator with steam followed by the rapid release of the steam through a quick opening valve. This method is typically referred to as a “high pressure discontinuous steamblow”. A second method practiced for nearly the last twenty years involves the continuous discharge of low pressure/high velocity steam from the steam generators through the steam piping. This method is typically referred to as a “low pressure continuous steamblow”.
Still other methods of the art involve:
(a) the chemical cleaning of the steam generator and its associated piping before the steam generator is repeatedly pressurized with high pressure air that is released through a quick opening valve. [This process is typically referred to as an “air blow cleaning”];
(b) a high velocity water flush of the steam circuits, followed by a chemical cleaning typically using EDTA followed by an extended steamblow through the steam system bypass valves to the condenser. This method is typically referred to as the “Siemens Augmented Bypass Operation”; and
(c) the chemical cleaning of the steam generator, followed by the high velocity flushing of the steam path from the steam generator through the superheater, in addition to the high pressure water “hydro-milling” of the steam piping to the steam turbine. This method may also include a steamblow upon commissioning of the steam generator to the consenser to confirm the absence of particulate contamination. This method is referred to as the “LARCOM Process”.
In yet other variations of the above methods, a combination of chemical cleaning of the steam generator and mechanical cleaning of the steam piping followed by a steamblow through the plant steam bypass system to the condenser have been practiced. The mechanical cleaning of the steam piping may include the abrasive blasting of the pipe interior, hydro milling with high pressure water or other similar practices.
While all of the above prior art methods may be successful in the removal of particulate contamination from the steam generator and pipe delivering steam to the steam turbine, the above prior art methods do not integrate the cleaning practices into other commissioning activities of the plant. They do not integrate the removal of particulate contamination with the removal of other types of steam contamination such as silica, cation conductivity and salts. Further the prior art cleaning methods typically limit the scope of the particulate debris cleaning effort to the steam generator, its associated piping and the steam piping. Little if any effort is made to remove particulate and other contamination found in the condenser. In these prior art methods, the condenser may be flushed with water prior to initial operation of the steam generator. Contamination removed from the condenser may also be removed by use of condensate polishing beds following initial steam discharge to the condenser.
Many of the prior art methods rely on the chemical cleaning of the steam generator and associated piping to reduce the potential for solid particle contamination. Many of the chemical cleaning solutions contain sodium, organic acids, organic corrosion inhibitors and other salts. Past experience has shown that it requires extensive post chemical cleaning flushing to remove all residual salts that may “hide-out” in systems that have been chemically cleaned. These residues from the chemical cleaning may add to the level of sodium, cation conductivity and salts in the steam cycle of the plant. The chemical cleaning processes also generate large volumes of waste solution and waste flush water.
Prior art methods make little effort to optimize conditions that will increase the rate of silica, elements that contribute to cation conductivity and other salt contaminants. Removal of these contaminants is typically left to the initial operation of the steam generator with steam bypassed to the condenser. The operation of a plant in steam bypass mode to the condenser is not optimized for the removal of silica and other contaminating salts in the steam. Steam bypassed to the condenser is cooled by addition of condensate to meet the design enthalpy limits of the condenser. Cooling of the bypass steam will result in the precipitation of silica dissolved in the steam. In the prior art, the clean-up of the steam condensate to remove silica and other non-particulate contamination during turbine bypass operation is out of the scope of the existing steam system cleaning practices used to remove particulate contamination. In the prior art, before steam can be discharged to the condenser through the bypass valves, the steam path must be cleaned to remove particulate contamination that may otherwise damage those valves.
Many prior art methods are also highly dependent on vigilant maintenance of the cleanliness of the systems cleaned prior to the initial operation of the steam generator. Corrosion following chemical cleaning, hydro-milling or other forms of mechanical cleaning may result in the formation of new particulate and salt contamination in the systems previously cleaned. Introduction of particulate contamination is also possible due to required mechanical work following many of the prior art multi-stage cleaning methods. Many prior art systems require extensive use of welding to close access point required for the cleaning methods. Use of corrosion inhibitors or other agents to preserve the cleaned surfaces may only exacerbate to amount of salts and organic compounds that will contaminate the steam during initial operation of the steam generator in bypass mode to the condenser.
The prior art practices of “discontinuous high pressure steamblow” and the “low pressure continuous steamblow” exhaust all steam from the steam cycle during the cleaning process, these methods require large volumes of high quality water, may take weeks to complete, and can result in environmental issues such as noise, related to the discharge of large volumes of steam. In addition, these methods do not generate conditions highly favorable to the removal of silica, elements that result in elevated cation conductivity or other non-particulate steam contamination.
These methods also do not involve the condenser and therefore do nothing to address contamination that might originate from the extensive amount of contaminated metal surface area in the condenser. These practices also require operation of the steam generator for an extended period of time with little possibility for other normal commissioning activities to be performed. The fuel consumed during such steamblow operations represents a significant expense. Extended operation of gas turbines of combined cycle power plants before tuning of the burners to minimize emissions during prior art methods also results in significant exhaust emissions.
In addition to the above, the prior art practices make only limited provisions for the assured protection of the condenser during the discharge of steam to the condenser. Specifically, none of the prior art practices make adequate provisions to prevent the continued discharge of steam to the condenser should the condenser lose coolant flow or should attemperation water used to cool the steam being discharged to the condenser be interrupted.
Prior art practices that rely on operation of the steam system in bypass mode through the bypass valve do not provide an assured means of protecting plant bypass valves from contamination or conditions that may cause the bypass valve to not close fully when plant conditions would require such closure to protect the condenser from damage. These same prior art methods make no provision to insure that the conditions generated by operation of the steam generation system in the bypass mode will optimize conditions for removal of either solid particle or other types of steam contamination due to the high pressure drop normally associated with typical bypass valves and the high back pressure of typical steam distribution sparge tubes in the condensers. Although some prior art provides for the installation of sacrificial tubes in the top layer of the condenser tube bank, none of the prior art cleaning methods provide for an assured means of shielding condenser tubes from high velocity particles or moisture droplets discharged into the condenser at high velocities.
Most prior art steam system cleaning methods do not provide for the comprehensive treatment of the steam condensate generated in the condenser during bypass mode operation. As a result, particulate contamination found in this condensate will result in the plugging of condensate and boiler feedwater pump strainers. No particulate contamination such as silica and dissolved salts are returned to the steam generators. Use of ion exchange beds to remove salt ions is expensive and is rarely applied.
Further, the prior art methods do not provide a means of cleaning the steam exhausted from the steam generator and steam piping to remove contamination prior to the admission of the steam to the condenser. Contamination transported by the initial steam discharged to the condenser will foul the condenser and may result in mechanical damage to the condenser due to the high velocity impingement of particulate contamination and entrained condensate droplets on the thin wall condenser tubing. Although some prior art methods provide for the installation of a metal target plate into the bypass steam circuit to the condenser to monitor the presence of particulate contamination, none of the prior art methods make provision for the diversion of contaminated steam away from the condenser when particulate contamination is indicated by such targets. The prior art also makes no provision for diversion of steam with high concentrations of non particulate contamination until such time as the level of contamination of the steam has been reduced to an acceptable level.
The prior art methods also make no special mechanical or chemical provisions during the operation of the steam generation equipment in bypass mode to create conditions that would enhance the removal of solid particle and other forms of steam contamination from the steam/condensate cycle.
In addition, prior art methods have not been practiced to generate physical and chemical conditions in the steam path that will insure the formation of a passive coating on the metal surfaces consistent with that formed by long term operation of the steam system at normal operating conditions.